1. Field of the Invention
The present invention relates to a paste applicator for discharging a paste onto a substrate through a nozzle and drawing a desired paste pattern on the substrate and, more particularly, to a paste applicator of the type that measures a gap between the nozzle and the substrate by a distance sensor and adjusts the gap between the nozzle and the substrate in accordance with the measured data of the distance sensor.
2. Description of the Prior Art
A paste applicator is an apparatus for drawing a paste pattern on a substrate by using a nozzle through which a paste is discharged, and employed in various fields as a drawing apparatus suitable for resistors of hybrid printed circuits, panel sealants of liquid crystal displays, etc.
In, for example, Japanese Patent Unexamined Publication No. 2-52742 an applicator is proposed wherein a paste, filled in a paste reservoir tube, is discharged onto a substrate through a nozzle provided at a distal end of the paste reservoir tube, and the nozzle and substrate are relatively moved in the horizontal direction by an X-Y table or the like, thereby drawing a paste film in a desired pattern on the substrate. The disclosed paste applicator includes a distance sensor capable of measuring a gap between the nozzle and the substrate, with the gap between the nozzle and the substrate being adjusted in accordance with measured data of the distance sensor to prevent irregularities of the pattern formed.
In the above conventional paste applicator, however, because the nozzle is provided at the center of the lower end of the paste reservoir tube and the gap is measured at a position just below the center of the distance sensor provided in side-by-side relation to the paste reservoir tube, a discharge port at the nozzle end and the measuring position of the distance sensor cannot be aligned and are offset with respect to each other in the horizontal direction. Accordingly, if undulations or fine irregularities are present on the substrate surface, an error is caused between the data of measurement detected by the distance sensor and the actual nozzle-to-substrate gap thereby requiring a complicated compensation means to correct such error.
Further, because the nozzle is provided at the lower end of the paste reservoir tube, if a paste of low viscosity is used, a further problem arises in that the paste is discharged through the discharge port at the nozzle end by gravity just after the paste is filled in the paste reservoir tube and the paste is applied to those locations where it is not to be drawn.
Additionally, the following problem has been experienced when liquid crystal sealing films are drawn on substrates for recent high-accurate liquid crystal displays by using the above conventional technique.
Namely, a new problem has been found relating to the above conventional discharge drawing technique that, when the measuring position of the distance sensor passes over an already drawn paste pattern with a certain thickness in the course of gap measurement, the nozzle and the substrate cannot be held at the desired gap, i.e., the nozzle height cannot be positioned at a set value of height, in those portions where a paste film is to be newly formed, and the width or height of the sealing film is increased in those portions, making it impossible to draw the desired sealing film.